Revealing mistake: When Jackie and Fez are doing their Roller Disco routine, it becomes very obvious that doubles are used for many of the routines. Fez does a lot of his own stunts but Jackie does very few. While they do a decent job of hiding the stunt doubles, there are several times when you can see the faces of the doubles and it is obvious it is not Mila Kunis or Wilmer Valderrama. And you do not need slow motion or stop action to see the difference.
Revealing mistake: The scene where Donna and Eric are running towards each other, each shot of Donna and Eric individually shows them running past the same umbrella and surfboard. They were using the same section of the beach to shoot both Eric and Donna running.